This invention relates to a dual intake valve engine with the timing of one valve being more appropriate for high engine speed and including a controller for controlling the air flow to that valve dependent upon engine speed and air pressure in the intake system.
When tuning an engine for better performance, one variable the engine designer will work with is the intake valve open and close time, in relation to the crankshaft rotation. The valve timing is customarily measured in degrees of crankshaft rotation, either before top dead center (BTDC) or after top dead center (ATDC) or before or after bottom dead center (BBTC, ABDC).
Usually, on a four stroke automobile engine, a camshaft controls the intake valve operation. Lobes on the camshaft apply pressure to the top of the valve stem and cause the valve to open and close as the camshaft turns. The camshaft is driven in coordination with the crankshaft of the engine, usually through belts or gears, and is synchronized with the crankshaft so that the valve opens and closes at the desirable time of the Otto cycle.
It is known that the optimum camshaft profile and valve timing specification varies according to the speed of the engine. When the engine is operating at a low speed, i.e. low rpm, such as when idling, the optimum camshaft profile is close to the theoretically normal opening and closing points, for example 0 degree ATDC and 180 degrees ATDC on the intake cycle. This is in order to maintain a steady, smooth, and strong idle. Such camshaft and valve timing may be considered xe2x80x9cslow speedxe2x80x9d or xe2x80x9cconservative.xe2x80x9d However, at higher engine speeds, usually above 2000 rpm, because of inertial effects of the intake air as it moves faster, the optimum intake valve opening time is earlier, for example 15xc2x0 BTDC, and the optimum closing time is later, for example 220xc2x0 ATDC. This xe2x80x9chigh speedxe2x80x9d or xe2x80x9chotxe2x80x9d camshaft profile improves air scavenging and provides more torque and power at high rpm, but will cause rough idling and increased emissions at low speed. The effect of having the optimum versus non-optimum valve timing at any given engine speed can easily amount to a 10-20% or more change in the power output of the engine. Thus the engine designer is left with contradictory optimizations. Higher specific power outputs allow the designer to reduce the engine size, thus reducing costs and increasing fuel efficiency.
Current systems seek to correct this dilemma with various mechanisms. One such mechanism consists of various cams, levers, and pulleys that alter the relationship between the crankshaft rotation and the camshaft rotation at various speeds. By altering the rotational timing relationship between the crankshaft and the camshaft, the effective valve timing may be altered. Another system uses rocker arm and lifter arrangements that cause the intake valves to open and close at different times at different engine rpms. Such rocker arms and mechanisms have various mechanical portions to effect the changes in valve opening and closing timing. Such systems often offer only fixed step changes to timing requirements, for example a setting for low speed operation, and a setting for high speed operation. Thus there is some compromise between the optimum valve timing at any engine speed, and the actual valve timing available. Typical prior art systems are complex and costly such that they are found only on expensive automobiles.
Thus there is a need for an improved system for effectively changing intake valve timing over various engine speeds. Particularly, a system that is economical, does not use complex mechanisms, and can provide more fully effective variable intake valve timing.